The present invention relates to the field of optical fiber transmission, and more particular to optical regeneration for optical fiber transmission systems. It is particularly applicable to transmission systems for return-to-zero (RZ) signals, such as, for example, transmission systems for soliton signals. The RZ signal corresponding to a high logic value being transmitted is referred to as a xe2x80x9cpulsexe2x80x9d or a xe2x80x9conexe2x80x9d, and the absence of signal, corresponding to a low logic value being transmitted, is referred to as a xe2x80x9czeroxe2x80x9d.
It is known that soliton pulses or xe2x80x9csolitonsxe2x80x9d can be transmitted in a portion of an optical fiber that has abnormal dispersion. Solitons are pulse signals of sech2 waveform. With such a pulse waveform, the non-linearity in the corresponding portion of the fiber compensates the dispersion of the optical signal. Soliton transmission can be modeled in known manner by the non-linear Schrxc3x6dinger equation. Proposals have also been made for dispersion-managed soliton transmission systems. In such systems, a dispersion pattern is used that is repeated periodically over thousands of kilometers: a fiber having abnormal dispersion (positive dispersion) succeeds a fiber having normal dispersion (negative dispersion) which almost totally compensates the abnormal dispersion accumulated in the first type of fiber. The signals thus propagate in fibers having high local dispersion but ultimately they undergo mean dispersion that is very low. Such a transmission scheme makes it possible to reduce jitter effects significantly (because of the low mean dispersion), to reduce inter-channel collision effects significantly (because of the high local dispersion), to improve the signal-to-noise ratio, and to increase the spectrum efficiency of the system. Such a system is described, for example, in an article by N. J. Smith and N. J. Doran, Journal of Lightwave Technology, vol. 15, No. 10 (1997), p. 1808 et seq.
It has been proposed to subject soliton signals to synchronous modulation by a clock signal or xe2x80x9cclockxe2x80x9d to correct their time jitter. Intensity modulation is described, for example, in an article by H. Kubota and M. Nakasawa, IEEE Journal of Quantum Electronics, vol. 29, No. 7 (1993), p. 2189 et seq. An article by N. J. Smith and N. J. Doran, Optical Fiber Technology, 1, p. 218 (1995) proposes phase modulation.
One of the problems encountered in optical fiber transmission systems lies in the distortion to which the optical signals are subjected when they are generated, transmitted, or switched, or more generally, each time they are processed optically in the transmission system. Optical regeneration by intensity modulation aims to solve that distortion problem by applying to each bit a signal whose intensity is at a maximum in the center of the time window for the bit, and of low intensity at the edges of the time window.
Unfortunately, intensity modulation offers a solution that is not entirely satisfactory, in particular in the xe2x80x9czerosxe2x80x9d of RZ signals. Noise in the xe2x80x9czerosxe2x80x9d, i.e. noise in the time windows corresponding to zero bits, or to bits without signals, is not completely eliminated, in particular for transoceanic transmission systems.
C. R. Menyuk, xe2x80x9cStability of solitons in birefringent optical fibers. I: Equal propagation amplitudesxe2x80x9d, Optical Letters, vol. 12, No. 8, Aug. 1987, proposes a model of the effects of birefringence on the propagation of solitons in single-mode fibers. Birefringence, i.e. the difference in optical index between the two polarization directions of optical fibers, causes a soliton pulse to be separated into two pulses that propagate at different speeds in respective ones of the two polarization directions. That article by C. R. Menyuk shows that, beyond a certain power, the Kerr effect causes time stabilization of the soliton pulses, by phase cross-modulation between the solitons propagating in the two polarization directions. Thus, beyond a certain amplitude, the pulses propagating in both polarization directions move together. The threshold amplitude depends on the birefringence of the fiber.
N. N. Islam et al, xe2x80x9cSolitons trapping in birefringent optical fibersxe2x80x9d, Optics Letters, vol. 14, No. 18, Sep. 1989 proposes an experimental demonstration of trapping orthogonally-polarized solitons in birefringent optical fibers. That demonstration confirms the results announced in the above-mentioned article by C. R. Menyuk.
N. S. Islam, xe2x80x9cUltrafast all-optical logic gates based on solitons trapping in fibersxe2x80x9d, Optics Letters, vol. 14, No. 18, Nov. 1989, uses trapping of orthogonally-polarized solitons in a birefringent fiber to form logic gates, with inversion, exclusive-OR, or AND functions. At their inputs, those gates receive solitons polarized in respective ones of the polarization directions of a fiber. Those gates use the fact that trapping of the solitons results in the solitons that are propagating in the polarization directions coinciding in time terms. Spectrally, the two trapped pulses undergo a frequency shift of about 1 THz, i.e. about 8 nm at 1550 nm. A filter at the output of the fiber removes the solitons that have undergone such a frequency shift, and makes it possible to obtain an exclusive-OR gate. The filter passes only those solitons which have not undergone any frequency shift, i.e. only those which have propagated alone in the birefringent fiber. A filter at 0.5 THz or 1 THz from the initial frequency of the solitons makes it possible to obtain an AND gate.
The invention proposes a solution to the problem of noise in transmission xe2x80x9czerosxe2x80x9d. It is particularly applicable to transmission systems for soliton signals.
More precisely, the invention provides apparatus for limiting noise in the xe2x80x9czerosxe2x80x9d of optical RZ signals, comprising means for mutually spectrally offsetting the signals as a function of their intensity, and a filter for filtering the offset signals, the attenuation applied by the filter to the signals that correspond to a xe2x80x9czeroxe2x80x9d value being greater than the attenuation applied by the filter to the signals that correspond to a xe2x80x9conexe2x80x9d value.
In one embodiment, the attenuation applied by the filter to the signals corresponding to a xe2x80x9czeroxe2x80x9d value is at least 6 dB greater than the attenuation applied by the filter to the signals corresponding to a xe2x80x9conexe2x80x9d value.
Preferably, the means for spectrally offsetting the signals comprise a birefringent fiber. This fiber may have birefringence of greater than or equal to 1xc3x9710xe2x88x925.
In one embodiment, the filter is a notch filter centered on the center frequency of the RZ signals.
The apparatus may further comprise an amplifier whose output is connected to the offsetting means.
It is also possible to provide a second birefringent fiber situated downstream from the filter, and disposed with polarization axes that are aligned relative to the first fiber.
The invention further provides a method of limiting noise in the xe2x80x9czerosxe2x80x9d of optical RZ signals, said method comprising:
a step of mutually spectrally offsetting the signals as a function of their intensity; and
a step of filtering the offset signals, so that the attenuation applied by the filter to the signals corresponding to a xe2x80x9czeroxe2x80x9d value is greater than the attenuation applied by the filter to the signals corresponding to a xe2x80x9conexe2x80x9d value.
Preferably, the attenuation applied by the filter to the signals corresponding to a xe2x80x9czeroxe2x80x9d value is at least 6 dB greater than the attenuation applied by the filter to the signals corresponding to a xe2x80x9conexe2x80x9d value.
In one implementation, the offsetting step comprises injecting the signals into a birefringent fiber. This fiber advantageously has birefringence of greater than or equal to 1xc3x9710xe2x88x925.
It is also possible to provide a step of amplifying the signals before the offsetting step.
In one implementation, the method further comprises a step of passing the signals through a birefringent fiber after the filtering step.